1. Field of the Invention
The present invention relates to a liquid-crystal display (hereinafter LCD), more particularly, to an art for controlling the gray-scale level data required for achieving a gray-scale display on an active matrix type LCD in which frame modulation is implemented.
2. Description of the Related Art
With the advent of an LCD permitting multiple gray-scale displays, there is a growing demand for an increase in the number of gray-scale levels. Means for expressing gray scale are classified into an analog technique and a digital technique in terms of a driver for driving a liquid-crystal panel.
The analog technique is such that drive voltage is varied and amplified in analog form and then applied to a liquid crystal. An analog signal is handled as it is. The analog technique has the advantages that no limitation is imposed on the number of gray-scale levels, and that full-color display can be achieved. A drawback of the analog technique is that, since numerous operational amplifiers are needed, the circuitry is complex and the power consumption is relatively high.
The digital technique is such that one of a plurality of reference voltages fed to a driver for driving a liquid-crystal panel is selected and applied to a liquid crystal. Compared with the analog technique, the digital technique has simple internal configuration. An IC used as a driver can therefore be manufactured at low cost. However, the number of internal elements increases with an increase in the number of gray-scale levels. The number of signals fed externally and the number of supplied reference voltages increase accordingly. This poses a problem in that the number of gray-scale levels (that is, the number of display colors) is relatively small.
A frame modulation technique has been used as a means for increasing the number of display gray-scale levels without increasing the amount of voltage associated with gray-scale levels and handled by a digital driver. In frame modulation, a drive voltage that differs among a plurality of frames is applied to pixels (that is a liquid crystal) in order to make intermediate brightnesses discernible. According to this technique, the number of gray-scale levels can be increased relatively effortlessly. FIGS. 1a to 1c show an example of the frame modulation technique. FIG. 1a shows a waveform of voltage to be applied to a liquid crystal for 15-level gray-scale display. FIG. 1b shows a waveform of voltage to be applied to a liquid crystal for 14-level gray-scale display. In the case of FIG. 1a, the fifteen gray-scale levels correspond to different drive voltage levels; +7, +6, etc., 0, etc., -6, and -7 V. In the case of FIG. 1b, the fourteen gray-scale levels correspond to different drive voltage levels; +7, +6, etc., 0, etc., -5, and -6 V. FIG. 1c shows a change in brightness relative to drive voltage.
According to a conventional frame modulation technique, flickers triggered by the display of a specific display pattern pose a significant problem. The flickers are attributable to the fact that when a pixel potential differs among frames, the transmittance of a liquid-crystal panel varies to bring about a change in brightness.
For driving a liquid-crystal panel, a lateral line inversion driving method, a vertical line inversion driving method, or a dot inversion driving method for handling adjoining dots has been adopted in an effort to eliminate flickers. These driving methods are devised because a liquid crystal must be driven with alternating voltage. Mention will be made of a liquid-crystal panel on the assumption that thin film transistors (hereinafter TFTs) are used to construct the liquid-crystal panel.
In the TFT type liquid-crystal panel shown in FIG. 2a (in FIG. 2a, Q denotes a transistor), even if a voltage having the same level in both positive and negative directions as shown in FIG. 2b (which is comparable to a drain potential of the TFT) is applied, a pixel potential (comparable to a source potential of the TFT) shifts to the negative side due to the capacitance C.sub.GS of a parasitic element. Even if the potential of an opposed electrode CEL is regulated, the drive voltage cannot be retained at either positive or negative polarity at all the pixels. This causes flickers. In FIG. 2a, SL denotes a scan line, DL denotes a data line linked to the drain of each TFT, C.sub.LC denotes the capacitance in a liquid crystal which shall be called liquid-crystal capacitance and P denotes the equivalent of pixel electrodes with a liquid crystal between them.
For suppressing the occurrence of flickers attributable to the capacitance of a parasitic element, it is useful to reverse the polarity of the writing voltage for each dot (pixel). When a large area of a liquid-crystal panel is viewed, it is seen that a difference in brightness between frames is averaged. Thus, the occurrence of flickers is suppressed. In the aforesaid lateral line inversion or vertical line inversion driving, the polarity of the writing voltage is reversed for each lateral line or vertical line. In dot inversion driving, the polarity of the writing voltage is reversed, in a zigzag form, for each dot.
As far as a normal display pattern used for character display or non-periodic graphic display is concerned, the aforesaid driving methods pose no problem. However, when a specific display pattern used for a periodic graphic display or for the display of many vertical and lateral straight lines, flickers occur.
As shown as an example in FIG. 3, when data having a display pattern coincident with the reversal in polarity of the drive (writing) voltage from the upper and lower drivers is displayed, since the logical states of rendered dots specified in the data (or the polarities of glowing pixels) are the same, the brightness is not averaged. This causes flickers. This display pattern will be referred to as a "same-polarity pattern" for convenience.
Frame modulation is a technique for varying a pixel potential (voltage applied to a liquid crystal) intentionally from frame to frame. When a high voltage is applied to all pixels associated with dots during a first frame and low voltage is applied to all the pixels during a second frame, a difference in brightness occurs between the first and second frames. An approach for coping with this problem is to average the brightness by mixing high voltages and low voltages during each frame. A pattern according to which the brightness is averaged will be referred to as an "averaged pattern" for convenience.
As in the same-polarity pattern, when the averaged pattern is consistent with a display pattern as shown in FIG. 4, flickers occur. Even if the averaged patterns are changed periodically, the periodicity is discernible as flickers to the human eye.
As mentioned above, in an LCD using known frame modulation, the occurrence of flickers cannot be avoided relative to a certain dot array specified by display data.